This invention relates generally to apparatus for aligning a mask with a workpiece and exposing a photosensitive-film-bearing surface of the workpiece in accordance with a pattern of the mask. More particularly, this invention relates to a step-and-repeat projection alignment and exposure system for successively aligning a pattern of a photomask with a plurality of different regions of a photosensitive-film-bearing surface of a semiconductive wafer and exposing the photosensitive film on each of these surface regions in accordance with the pattern of the mask.
In most conventional optical alignment and exposure systems for aligning and exposing a photosensitive-film-bearing surface of a semiconductive wafer in accordance with a pattern of a photomask, the photosensitive-film-bearing surface of the wafer and the pattern-bearing surface of the mask are brought into intimate contact to establish parallel-plane alignment therebetween. The wafer and the mask are then separated to permit out-of-contact alignment of the pattern of the mask and the photosensitive-film-bearing surface of the wafer. After this pattern alignment operation, however, the photosensitive-film-bearing surface of the wafer and the pattern-bearing surface of the mask are again brought into intimate contact and so maintained while the photosensitive film on the surface of the wafer is exposed through the mask. Optical alignment and exposure systems of this type are shown and described, for example, in U.S. Pat. No. 3,192,844 issued July 6, 1965, to Peter R. Szaz et al. and in U.S. Pat. No. 3,220,231 issued Nov. 30, 1965, to James A. Evans et al. The mechanical abrasion inevitably produced between the photosensitive-film-bearing surface of the wafer and the pattern-bearing surface of the mask in such optical alignment and exposure systems may scratch or otherwise damage these surfaces. Scratched or otherwise damaged areas on the pattern-bearing surface of the mask are especially undesirable since they will be reproduced on all wafers with which the mask is subsequently used. Moreover, the resolution that can be obtained with conventional alignment and contact printing systems of this type is limited to lines of about 0.1 mil or more in width spaced about 0.1 mil or more apart.
A projection alignment and exposure system for eliminating mechanical abrasion between the photosensitive-film-bearing surface of the wafer and the pattern-bearing surface of the mask has now been developed by AEG-TELEFUNKEN Company of Germany. The resolution that can be obtained with this system is limited to lines of about 5.0 microns or more in width spaced about 5.0 microns or more apart. Neither this projection alignment and exposure system nor the above-described contact printing alignment and exposure systems can be employed to step and repeat the pattern of the mask on different regions of the photosensitive-film-bearing surface of the same wafer and to subsequently step and align the patterns of other masks with the pattern previously formed on each of these same surface regions of the wafer.
In all of the above-described optical alignment and exposure systems the wafer chuck, the mask holder, or both are rigidly supported on a base unit of the system so that external forces acting upon the base unit may also affect the parallel-plane and pattern alignment established between the photosensitive-film-bearing surface of the wafer and the pattern-bearing surface of the mask. Moreover, the wafer chuck is typically mounted for translational and rotational movement in a horizontal plane substantially parallel to the pattern-bearing surface of the mask, as well as, for translational movement along a vertical axis toward and away from the pattern-bearing surface of the mask and pivotal movement about the vertical axis. This tends to impair the stability of the wafer chuck and may therefore also affect the parallel-plane and pattern alignment established between the photosensitive-film-bearing surface of the wafer and the pattern-bearing surface of the mask, thereby degrading the resolution that may be achieved. The foregoing factors limit the usefulness of such optical alignment and exposure systems in fabricating smaller and higher density integrated circuits, or the like, and in high yield production set-ups.
Accordingly, one of the principal objects of this invention is to provide an improved optical alignment and exposure system for aligning a pattern of a mask with a photosensitive-film-bearing surface of a wafer and exposing the photosensitive film on the surface of the wafer in accordance with the pattern of the mask without ever bringing the wafer into contact with the mask and with a resolution that may be extended to lines of 1.0 micron or less in width spaced 1.0 micron or less apart.
Another of the principal objects of this invention is to provide an improved optical alignment and exposure system for aligning a pattern of a mask with a plurality of different surface regions of a wafer and exposing a photosensitive film on each of these surface regions in accordance with the pattern of the mask and for subsequently aligning patterns of other masks with each of these surface regions and exposing photosensitive films thereon in accordance with the patterns of these other masks.
Still another of the principal objects of this invention is to provide an improved optical alignment and exposure system for minimizing the effect of external forces upon the parallel-plane and pattern alignment established between a pattern of a mask and a photosensitive-film-bearing surface of a wafer and for increasing the stability of the wafer chuck and minimizing the required movement thereof to help maintain the parallel-plane and pattern alignment established between the pattern-bearing surface of the mask and the photosensitive-film-bearing surface of the wafer.
These objects are accomplished according to the illustrated preferred embodiment of this invention by providing a step-and-repeat projection alignment and exposure system in which a wafer-bearing chuck, an annular reference plate, and a copy lens system are mounted within a floating unit along a horizontal optical axis and in which a mask-bearing holder is mounted on the floating unit along a vertical optical axis orthogonally intersecting the horizontal optical axis at an inclined mirror also mounted within the floating unit. These elements are arranged so that the copy lens system produces an image of a pattern-bearing surface of the mask at a plane positioned between the annular reference plate and the wafer chuck and further produces an image of a selected pattern-bearing region of the photosensitive-film-bearing surface of the wafer at the pattern-bearing surface of the mask. The wafer chuck is supported for pivotal movement about and translational movement along the horizontal optical axis so that an unused marginal portion of the photosensitive-film-bearing surface of the wafer may be moved forward into face-to-face abutment with an annular reference surface of the annular reference plate to orient the photosensitive-film-bearing surface of the wafer in parallel-plane alignment with the image plane of the pattern-bearing surface of the mask and so that the selected pattern-bearing surface region of the wafer may then be moved backward into the image plane of the pattern-bearing surface of the mask. A stop mechanism movable with the wafer chuck may be clamped to the floating unit when the photosensitive-film-bearing surface of the wafer is in face-to-face abutment with the annular reference surface of the annular plate to automatically stop the wafer chuck when the selected pattern-bearing surface region of the wafer is substantially positioned in the image plane of the pattern-bearing surface of the mask. Alternatively, a microscope of an optical unit pivotally mounted on a base unit and positioned above the mask holder may be employed for viewing the pattern-bearing surface of the mask and the image of the selected pattern-bearing surface region of the wafer while the wafer chuck is manually adjusted to precisely position the selected pattern-bearing surface region of the wafer at the image plane of the pattern-bearing surface of the mask. The optical unit is provided with a rotatable turret for positioning a selected objective lens system of the microscope in optical alignment with a stationary occular lens system of the microscope during this wafer-focusing and subsequent wafer-focusing and pattern-alignment operations. Three beams of illuminating light are projected through the mirror and along the horizontal optical axis so that only three spaced portions of the photosensitive-film-bearing surface of the wafer are illuminated during these wafer-focusing and pattern-alignment operations.
The mask holder is mounted for translational and rotational movement in a horizontal plane orthogonally intersecting the vertical optical axis so that after the above-described wafer-focusing operation the mask holder may be adjusted to align the pattern of the mask with the image of the pattern of the selected surface region of the wafer while they are viewed through the microscope of the optical unit. The floating unit is floatingly supported, for example, on three balls captivated within the base unit so that the floating unit and, hence, the wafer chuck and the mask holder together as a unit, may be scanned relative to the microscope of the optical unit to facilitate and verify alignment of all portions of the pattern of the mask with the image of the pattern of the selected surface region of the wafer. Mounting the floating unit in this manner also prevents external forces acting on the base unit from affecting the floating unit and, hence, the parallel-plane and pattern alignment established between the pattern of the mask and the image of the pattern of the selected surface region of the wafer. The floating unit is also coupled to the base unit by a parallelogram linkage to prevent the floating unit from rotating relative to the base unit. A clamping mechanism comprising, for example, a pair of vacuum cups secured to the base unit and spring biased into sliding abutment upon a pair of plates secured to the floating unit may be employed to clamp the floating unit in any position to which it may be scanned, whereupon the mask holder may be adjusted to further align the pattern of the mask with the image of the pattern of the selected surface region of the wafer. Following this pattern alignment operation, the floating unit is unclamped and a centering mechanism employed to automatically move the floating unit to a central position at which an exposure lens system of the optical unit will be aligned with the vertical optical axis upon being rotated by the turret to an operative position for receiving a beam of exposure light. The centering mechanism may comprise, for example, three roller bearings pivotally secured to the base unit and normally clustered within an enlarged cylindrical aperture of the floating unit to permit movement of the floating unit relative to the base unit, but pivotable outward into engagement with the wall of this enlarged cylindrical aperture to force the floating unit to the central position.
A loading arm is mounted on a movable stage secured to the floating unit above the wafer chuck and is supported for angular movement in a vertical plane orthogonally intersecting the horizontal optical axis. The movable stage may be stepped to a plurality of different positions arranged in orthogonal rows and columns. When it is stepped to a central position, the loading arm may be employed to pick up a wafer at a loading station and deposit the wafer upon the wafer chuck in a central position so that a central pattern-bearing surface region of the wafer may be aligned and a photosensitive film thereon exposed in accordance with the pattern of the mask. Following these alignment and exposure operations, the loading arm may be employed to pick up the wafer from the wafer chuck, the movable stage stepped to a different position, and the loading arm employed to redeposit the wafer upon the vacuum chuck in a different position so that a different pattern-bearing surface region of the wafer may be aligned and a photosensitive film thereon exposed in accordance with the pattern of the same mask. This may be repeated until the last pattern-bearing surface region of the wafer has been aligned and the photosensitive film thereon exposed in accordance with the pattern of the mask. The loading arm is then employed once again to pick up the wafer from the wafer chuck, after which the movable stage is again stepped to the central position and the loading arm employed to return the wafer to the loading station. Subsequently, the loading arm and the movable stage on which it is mounted may be similarly employed to align each of the previously aligned pattern-bearing surface regions of the wafer and expose photosensitive films thereon in accordance with the patterns of other masks.
Other and incidental objects of this invention will become apparent from a reading of this specification and an inspection of the accompanying drawings.